1. Field of the Invention
The present invention relates to a wet friction plate comprising a plurality of friction materials disposed circumferentially on a surface of an annular first plate with a plurality of radial oil passages each provided between the adjacent friction materials and extending radially, the friction materials being in contact with a surface of an annular second plate to transmit torque.
2. Description of the Related Art
A conventional shifting clutch of an automatic transmission employs a wet hydraulic clutch. The wet hydraulic clutch comprises a plurality of friction plates and a plurality of separator plates, which are superposed alternately on one another, and brought into pressure contact with one another by hydraulic pressure, to thereby transmit torque. For absorbing a frictional heat generated when the hydraulic clutch is shifted from the non-engaged state to the engaged state and preventing wear of the friction materials, lubricating oil is supplied to contact surfaces of the friction plates and the separator plates.
In order to enhance the responsiveness of the hydraulic clutch, the distance between the friction plates and the separator plates is set to be small. Also, in order to ensure a sufficient torque-transmitting capacity in the engaged state of the hydraulic clutch, the total area occupied by oil passages on the friction plates is limited. Therefore, the following problem occurs: The lubricating oil remaining between the friction plates and the separator plates is difficult to discharge in the non-engaged state of the hydraulic clutch, and a drag torque of the lubricating oil is generated by the relative rotation of the friction plates and the separator plates.
A friction plate is known from Japanese Patent Application Laid-open No. 4-194422, in which oil reservoirs are formed in inner peripheral edges of friction materials on the friction plate to retain a lubricating oil therein, thereby preventing the lubricating oil from flowing between the friction plate and a separator plate to reduce the drag torque, and discharging, by centrifugal force, the lubricating oil accumulated in the oil reservoirs through oil passages extending radially.
The friction plate described in the above-described Japanese Patent Application Laid-open No. 4-194422 suffers from not only an increased processing or working cost, because the oil reservoir is required to be formed in the inner peripheral edges of the extremely thin friction materials, but also from shedding of the friction materials, because the lubricating oil is liable to flow from the oil reservoirs into a space between each of the friction materials and a plate supporting the friction materials.
The present invention has been accomplished with the above circumstances in view, and it is an object of the present invention to provide a further reduction in friction resistance in the non-engaged state of a hydraulic clutch or hydraulic brake of an automatic transmission.
To achieve the above object, there is provided a wet friction plate comprising a plurality of friction materials disposed circumferentially on a surface of an annular first plate with a plurality of radial oil passages each provided between adjacent ones of the friction materials and extending radially. The friction materials are in contact with a surface of an annular second plate to transmit torque. The plurality of friction materials are disposed at a plurality of radially inner and outer stages with circumferential oil passages provided between the stages and extending circumferentially along inner peripheral edges of the friction materials, and at least a portion of each of the inner peripheral edges of the friction materials is inclined radially outwards toward a radially inner end of each of the radial oil passages facing a delayed side of each friction material in a rotational direction.
With the above arrangement, at least a portion of the inner peripheral edge of each of the friction materials mounted on the surface of the annular first plate is inclined outwards toward the radially inner end of each of the radial oil passages facing a delayed side of each friction material in the rotational direction. Therefore, the lubricating oil flowing outwards by centrifugal force and flowing toward the delayed side in the rotational direction by the Coriolis force can be guided on at least a portion of each of the inner peripheral edges of the friction materials and introduced effectively to the inner ends of the radial oil passages on the delayed side in the rotational direction. Thus, the amount of the lubricating oil flowing onto the surfaces of the friction materials in the entire region of the surface of the first plate and remaining between the surfaces of the first and second plates can be reduced, to thereby prevent an increase in friction resistance due to the dragging of the lubricating oil. Also, the first and second plates can be moved uniformly away from each other by a hydraulic pressure load of the lubricating oil flowing into the radial oil passages, to thereby prevent an increase in friction resistance further effectively. Moreover, because the plurality of friction materials are disposed at the plurality of radially inner and outer stages with the circumferential oil passage provided between the inner and outer stages to extend circumferentially, a hydraulic load of the lubricating oil can be generated in both the inner radial oil passages and the outer radial oil passages, whereby the first and second plates can be moved further reliably away from each other.
In addition to the arrangement described above, there may also be provided a wet friction plate wherein the radial oil passages defined between the friction materials at the radially inner stage are offset to the delayed side in the rotational direction with respect to the radial oil passages defined between the friction materials at the radially outer stage.
With the above arrangement, the inner radial oil passages are offset to the delayed side in the rotational direction with respect to the outer radial oil passages. Therefore, the lubricating oil flowing out of the inner radial oil passages can be guided to the inclined inner peripheral edges of the outer friction materials and permitted to further smoothly flow into the outer radial oil passages located on the delayed side in the rotational direction.
In addition to the arrangement described above, there may also be provided a wet friction plate wherein the inner peripheral edges of each of the friction materials are inclined radially outwards to form a V-shape or U-shape projecting toward radially inner ends of a pair of the radial oil passages facing circumferentially opposite sides of each of said friction material.
With the above arrangement, in addition to the inner radial oil passages being offset to the delayed side in the rotational direction with respect to the outer radial oil passages, the inner peripheral edges of each of the friction materials are inclined radially outwards to form the V-shape or U-shape. Therefore, even when the first plate is rotated in either direction, the lubricating oil can be guided on the delayed side of the inner peripheral edge of each friction material inclined into the V-shape or U-shape and permitted to smoothly flow into the outer radial oil passages located on the delayed side in the rotational direction.